Scilab Function
Last update : 12/1/2004

analpf - create analog low-pass filter

### Calling Sequence

  `[hs,pols,zers,gain]=analpf(n,fdesign,rp,omega)`

### Parameters

- `n`: positive integer : filter order
- `fdesign`: string : filter design method : `'butt'` or `'cheb1'` or `'cheb2'` or `'ellip'`
- `rp`: 2-vector of error values for cheb1, cheb2 and ellip filters where only `rp(1)` is used for cheb1 case, only `rp(2)` is used for cheb2 case, and `rp(1)` and `rp(2)` are both used for ellip case. `0<rp(1),rp(2)<1`
  - `-` for cheb1 filters `1-rp(1)<ripple<1` in passband
  - `-` for cheb2 filters `0<ripple<rp(2)` in stopband
  - `-` for ellip filters `1-rp(1)<ripple<1` in passband `0<ripple<rp(2)` in stopband
- `omega`: cut-off frequency of low-pass filter in Hertz
- `hs`: rational polynomial transfer function
- `pols`: poles of transfer function
- `zers`: zeros of transfer function
- `gain`: gain of transfer function

### Description

Creates analog low-pass filter with cut-off frequency at omega.

`hs=gain*poly(zers,'s')/poly(pols,'s')`

### Examples

    //Evaluate magnitude response of continuous-time system
    hs=analpf(4,'cheb1',[.1 0],5)
    fr=0:.1:15;
    hf=freq(hs(2),hs(3),%i*fr);
    hm=abs(hf);
    plot(fr,hm)

### Author

C. B.